I had that on my 68 after having it worked on at a local garage. I had put some shims on one side of each U bolt that holds the back axle on to correct the pinion angle. The mechanic saw them and didn't know what they were for so he took them out. He neglected to tell me about it and he also neglected to tighten the U bolts back, leaving slack that allowed the axle to rotate a little under torque. Got the clunk every time I shifted. Once I figured it out it was an easy fix.
